Fix layout issues caused by having more than two panels within a dock row.
Fix dockable views "slipping" off the user's mouse cursor.
Correctly calculate target position
Indicate the number of each panel so I can tell them apart
Enable space reservation.
Ensure space reservations do not corrupt the top margin.
Stick dock items at the end of dock rows when the position isn't specified.
Properly hit-test the space reservations.